1. Field of the Invention
The present invention relates to a supercharged engine and more particularly to a supercharged engine having main and sub throttle valves for a motor vehicle.
2. Description of the Related Art
In general, a supercharged engine is used as a driving source of a motor vehicle and is located at one end of an intake passage. An air cleaner is located at the other end of the intake passage. A mechanical supercharger is located in the intake passage. A throttle valve is located at a downstream side of the mechanical supercharger in the intake passage.
In the above-mentioned supercharged engine, when the throttle valve is suddenly shut, for example when the motor vehicle speed is reduced, the pressure between the mechanical supercharger and the throttle valve in the intake passage is suddenly increased. As a result, the high pressure caused in the intake passage has a negative influence on the mechanical supercharger and throttle valve.
For the prevention of such drawback, in a supercharged engine shown in Japanese Patent Laid-open No. 62(1987)-29734, a main throttle valve is located at a downstream side of a mechanical supercharger in an intake passage, and a sub throttle valve is located at an upstream side of the mechanical supercharger in the intake passage.
In the above construction, when the main throttle valve is closed, the sub throttle valve is also closed at the same time. Thus, intake air is not supplied to the mechanical supercharger, so that the pressure between the mechanical supercharger and the main throttle valve in the intake passage is not increased. However, a large negative pressure is caused between the sub throttle valve and the mechanical supercharger in the intake passage. Therefore, the large negative pressure also negatively influences the mechanical supercharger and the sub throttle valve.